Charge pumps are commonly used for converting from one DC voltage level to another by making use of a plurality of capacitors that are charged up and then connected in parallel. The principle is best described by referring to a simple schematic representation of a charge pump circuit as shown in FIGS. 1 and 2. FIG. 1 shows a plurality of capacitors (in this case three capacitors) 100, 102, 104 connected in parallel across a DC voltage source 108. Switches 110, 112, 114, 116, 118, 120, 122, 124, 126, 128 are provided to selectively open and close to allow the capacitors to be charged in parallel by the voltage source 108, and then be connected in series with the voltage source to provide the effective circuit shown in FIG. 2 (switches 110–126 deleted for purposes of simplicity). This, in effect provides an output voltage across output capacitor 132 that is the sum of the voltages across each individual capacitor and the voltage of the voltage source (four times higher in this case for the three capacitors 100, 102, 104). In the simplified circuit of FIG. 1, the combined voltage on the capacitors 100, 102, 104 as well as the voltage of the voltage source 110 is applied across the output capacitor 132 by making use of the switch 128 working in conjunction with the other switches to charge the output capacitor 132 to four times the voltage of the voltage source 108. This concept is used to provide a DC—DC converter such as the prior art charge pump converter 300 shown in FIG. 3.
The converter of FIG. 3 takes the form of a pump circuit which includes switches and capacitors, including an output capacitor 304. The charge pump circuit is supplied by a DC voltage source 300, and includes an accumulation capacitor 302. The switches, in this case, comprise three MOS transistors, 310, 312, 314, the switching of which is controlled by signals from a clock circuit 320. During the charging portion, PMOS 310 and NMOS 312 are opened (turned on) to permit current to flow through them to charge up capacitor 302, while PMOS 314 is turned off. Once the capacitor 302 is charged, the clock circuit 320 biases the gates of the transistors to turn off transistors 310 and 312, and turn on transistor 314. This provides almost twice the input voltage across the output capacitor 304 due to the serial connection of the voltage source 300 and charged capacitor 302. As shown in FIG. 3, the output capacitor 304 is separated from the pump circuit by a diode 322. In order to support high charging currents the circuit of FIG. 3 makes use of rather large PMOS 310 and NMOS 312 transistors. This results in high parasitic capacitance, which, in turn, results in low charging rate.